Britney Spears Tweets Photo from Vegas Dress Rehearsal

If Britney Spears indulged in any holiday feasting this season, it certainly isn't showing.

In the wee hours of Friday morning, the pop singer Tweeted a sexy photo of herself rehearsing for her new Las Vegas show, "Piece of Me," opening Friday night.

"Can't believe the 1st show is almost here!!!" she says in the caption.

Fans got a behind-the-scenes glimpse of Spears prepping for her Vegas residency in the E! documentary, I Am Britney Jean, in which the pop icon confessed to being a poor match for the entertainment business.

"I have always been kind of shy, since I was a little girl," she says. "It's who I am to be modest, so I really can't help it. I turn into this different person … seriously, bipolar disorder … This is all I've know all my life – cameras, being followed, being part of the industry. … I'm not really made for this industry."

Spears also confessed to enjoying sex, but to having a new point of view on the subject now that she's in her 30s.

"I think sex is great," she said. "But I feel a little different about it now that I'm older. Sometimes I feel like I'm 20 and then sometimes I feel like I'm 50."